The present invention relates to a flexible tubular pipe for transporting fluids (especially gaseous hydrocarbons) and to be used in an offshore oil production field. It relates more particularly to a flexible pipe of the unbonded type for transporting gaseous hydrocarbons or two-phase hydrocarbons containing a gaseous phase.
These flexible pipes, which are formed from a combination of various superposed concentric layers, are called unbonded pipes whenever these layers have a certain freedom to move relative to one another. These flexible pipes comply with, among others, the recommendations of the normative document API 17J “Specification for Unbonded Flexible Pipe” published by the American Petroleum Institute. The constituent layers comprise especially polymeric sheaths, generally for providing a sealing function, and reinforcing layers intended to take up the mechanical forces and which are formed by windings of metal wires or strips or various tapes or sections made of composites.
Unbonded flexible pipes used most often in the offshore oil industry generally comprise, from the inside outward, an internal carcass consisting of an interlocking strip which serves to prevent the pipe from collapsing under the effect of the external pressure, a polymeric internal sealing sheath, a pressure vault consisting of at least one interlocking profiled metal wire wound helically in a short pitch, said pressure vault serving to take up the radial forces associated with the internal pressure, tensile armor plies formed by long-pitch helical windings of metal or composite wires, said armor plies being intended to take up the longitudinal forces undergone by the pipe, and finally an external sealing sheath intended to protect the reinforcing layers from seawater. Such a pipe is called a rough-bore pipe since the innermost element is the internal carcass that forms a rough bore owing to gaps between the turns of the carcass.
Flexible pipes of the smooth-bore unbonded type that do not have an internal carcass are also known, in which the innermost element is a smooth-walled impermeable polymeric tube.
Smooth-bore pipes are generally used for conveying fluids that do not have a gaseous phase. In practice, their use is reserved for water injection pipes. This is because, in the case of two-phase fluids, diffusion during the flow of gas along the polymeric internal sealing tube causes an increase in the pressure within the annular space lying around the internal sheath. When the pressure in the annular space becomes greater than the internal pressure in the pipe, such as for example when a decompression occurs following a production shutdown, the pressure difference may result in the internal polymeric sealing tube collapsing. It is, among other things, to avert this risk that an internal carcass is preferably placed inside said internal polymeric tube, which amounts to producing a rough-bore pipe. This is why it is general practice to use rough-bore pipes for transporting gaseous or two-phase hydrocarbons.
One problem is apparent with rough-bore pipes for the production and exportation of gaseous hydrocarbons. This problem is due to the flow of gas in the pipe and more precisely to the formation of vortices that appear in contact with the gaps between the turns of the carcass. Specifically, the surface discontinuity encountered at these gaps leads to the formation of vortices that disturb the flow of gas in the pipe. These vortices induce cyclic pressure fluctuations that may lead to resonance phenomena (vibrations, noise) in the pipe and at the equipment and pipework located on the platform or the floating production support, and also in the submerged equipment. These pressure fluctuations, and most particularly these resulting vibrations, may become very substantial and lead to fatigue phenomena, especially in said equipment and at the ends of the pipes that then undergo larger stresses than those for which said pipes have been designed, which may result in leaks.
Patent application FR 2 856 131 discloses a first solution of this problem, consisting in lining the inside of the rough-bore pipe with a polymeric sheath pierced by holes.
Patent application WO 2004/005785 teaches a second solution consisting in providing the internal carcass of the rough-bore pipe with longitudinal through-passages, said through-passages making it easier for the gas to flow toward the inside of the turns of the carcass so as to prevent the formation of vortices at the gaps between turns.
These two solutions prove to be expensive and tricky to implement.
Patent applications FR 2 775 052, FR 2 846 395 and FR 2 852 658 disclose particular smooth-bore pipes that can transport gaseous hydrocarbons. These pipes include an intermediate impermeable polymeric sheath lying between the pressure vault and the tensile armor plies. In the event of the external sheath accidentally tearing, the external hydrostatic pressure is taken up by this intermediate sheath, which is itself supported by the pressure vault. This has the effect of protecting the internal sealing tube and of preventing it from collapsing on itself. In addition, the annular space lying between the internal sealing tube and said intermediate sheath is provided with a drainage layer for draining away the diffusion gases, said layer being intended to evacuate these gases along this annular space up to one of the two ends of the pipe. In FR 2 775 052, the drainage layer consists of a pair of armor plies wound around the pressure vault at an angle of greater than 35°. In FR 2 846 395 and FR 2 852 658, the drainage layer consists of specific sections that include recesses and are helically wound around the pressure vault with an angle of greater than 55°. The drainage layer makes it possible to limit the risk of overpressurizing the diffusion gases in the annular space and therefore the risk of the internal tube collapsing. However, these solutions do not satisfactorily solve the problem of safety and reliability when transporting gaseous hydrocarbons under very high pressure. This is in particular the case of export pipes intended to convey gas, purified and compressed beforehand, over large distances and at a very high flow rate. Under these severe conditions, the diffusion of gas through the sealing tube reaches a high level, thereby requiring very effective drainage means in order to avoid the risk of said tube collapsing. Moreover, if such an incident were to occur, for example following depressurization of the pipe, it would be very difficult to detect this. If ever the sealing tube were to collapse on itself and be damaged without this being detected, the pipe would run the risk of bursting upon repressurization, with particularly serious consequences.
FR 2 775 051 discloses a pipe intended to solve other problems, the structure of said pipe being of another type since the internal tube is metallic and corrugated, instead of being polymeric and smooth. Moreover, this document simply discloses the fact that the helix angles of the two pairs of tensile armors are less than (or equal to) 55°. In one particular example, it also teaches a combination of an angle greater than 35° for the inner pair of armors and an angle less than 30° for the outer pair of armors, which is completely different from the combination specific to the invention, which will be defined later.